Okay, was able to reproduce in a more scientific manner. smplayer doesn't have anything to do with it. The problem seems to have something to do with vdpau and fullscreen and possibly twinview. The test below starts by rebooting machine and then ran mplayer to watch a 30 minute video. Everything was fine the first time around, then I started stopping and restarting mplayer and switching back and forth from fullscreen.

Getting error 1 2335 everytime I go fullscreen on my secondary monitor. After stopping and restarting mplayer and going fullscreen 3 times, it then started giving me the 1 426 error and at that point I permanently lose vsync on the secondary monitor, vdpau is always using blit queue at that point. Only way to temporarily fix is to reboot. I'm using the Ubuntu repo version of the nvidia driver, the very latest version made no difference.

Attaching an nvidia bug report log which was run after getting the results pasted below.

Note in the tests below, I'm simply running the command line "mplayer FILE", and my config file looks like this: